Hydrogen gas is used and produced in many applications. Since the amount of hydrogen in a gas stream produced by a given process may be an indicator of system efficiency, the systems typically utilize sensors, such as combustible gas sensors to determine the level of hydrogen. An example of a prior art system having an arrangement for monitoring combustible gas is shown in FIG. 1A. The electrochemical system 12 receives water from an external source 14 and passes it through a deionizing bed 16. Once the water has been properly conditioned, it is supplied to an electrochemical cell 18 which disassociates the water into hydrogen and oxygen gas.
One example of an electrochemical cell 18 is a proton exchange membrane electrolysis cell which can function as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas, and can function as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity. Referring to FIG. 1B, which is a partial section of a typical anode feed electrolysis cell 100, conditioned water 102 is fed into cell 100 on the side of an oxygen electrode (anode) 116 to form oxygen gas 104, electrons, and hydrogen ions (protons) 106. The reaction is facilitated by the positive terminal of a power source 120 electrically connected to anode 116 and the negative terminal of power source 120 connected to a hydrogen electrode (cathode) 114. The oxygen gas 103 and a portion of the process water 108 exit cell 100, while protons 106 and water 110 migrate across a proton exchange membrane 118 to cathode 114. At cathode 114, hydrogen gas 112 is formed and removed. Water is also removed from cathode 114.
A typical fuel cell uses the same general configuration as is shown in FIG. 1B. Hydrogen gas is introduced to the hydrogen electrode (the anode in fuel cells), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in fuel cells). Water can also be introduced with the feed gas. The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, hydrocarbon, methanol, or any other hydrogen source that supplies hydrogen at a purity suitable for fuel cell operation (i.e., a purity that does not poison the catalyst or interfere with cell operation). Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water, which additionally includes any feed water that is dragged through the membrane to the cathode. The electrical potential across the anode and cathode can be exploited to power an external load.
In other embodiments, one or more electrochemical cells can be used within a system to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water as needed. Such systems are commonly referred to as regenerative fuel cell systems.
After the electrochemical cell 18 disassociates the water, oxygen and hydrogen gas exit the cell 18 through conduits 20 and 22 respectively. As mentioned herein above, in addition to the gas products, water entrained in the gases exits with the oxygen and hydrogen. The hydrogen conduit 22 typically connects with a hydrogen phase separator 24 which extracts most of the water from the gas, with the water exiting the phase separator 24 through a valving arrangement which recycles the water back into the electrochemical cell water feed conduit. Depending on the needs of the application, additional water may be removed from the hydrogen gas by passing through an optional desiccant gas dryer 26 before exiting the process for use in the application.
The oxygen gas stream 20 also enters into a phase separator 28 with a majority of the water separating from the gas stream and dropping to the bottom of the separator 28. As with the hydrogen separator 24 this water is removed and recycled into the electrochemical cell water feed conduit. The separated hydrogen gas exits the phase separator 28 via a conduit 32 to exit the process. Since it is desirable to monitor for the presence of hydrogen gas in the oxygen gas stream through an orifice 40 to a combustible gas sensor 36. A gas dryer 38, such as a NAFION tube dryer, is usually placed in line between the phase separator 28 and the sensor 36 to remove water still entrained in the gas. Unfortunately, since the gas stream can still have a relative humidity greater than 95%. This high relative humidity results in lower monitoring performance than is desired.
Accordingly, what is needed in the art is a system for monitoring combustible gas levels in a gas stream that reduces or eliminates the effects of relative humidity on the combustible gas sensor.